1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for limiting access to private cellular networks.
2. Description of Related Art
In today's highly advanced cellular radio communications systems, there is a growing need for network operators to be able to differentiate between numerous sets of different end users. Consequently, in order to meet this growing need, operators are deploying a multitude of relatively small private and/or closed cellular sub-networks within the larger, public cellular networks. As such, the use of such private sub-networks allows the public network operators to guarantee selected sets of end users a specified quality of service. Similarly, only specific end users are allowed to access cells belonging to the closed sub-networks. For example, an indoor radio network covering the offices of a particular business can be operated as a private or closed sub-network within a public cellular network. Consequently, only employees of that business would be allowed to access and use that indoor radio network.
Of crucial importance to the success of these new private and closed networks is that they can be readily integrated with public cellular networks. In particular, from the viewpoint of a private network subscriber, the overall system should appear to allow seamless handovers between the private and public networks' cells. Furthermore, it is also important to ensure that cellular users who are not subscribers to a private network (hereinafter "non-subscribers") should not be allowed to access (or even be aware of) the private network's cells.
FIG. 1 is a diagram that shows an exemplary mobile communications system, which provides radio coverage for mobile terminals (not explicitly shown) with a plurality of private network cells operating within an area covered by one or more macro-cells of a public cellular network. Hereinafter, for clarity, assume that a mobile end user can be a subscriber to a public and private network, but a "non-subscriber" is a mobile end user that does not subscribe to the private network(s) involved. Alternatively, hereinafter a "subscriber" is a mobile end user that subscribes to the private network(s) involved. In an actual operation, the public cellular network can comprise a plurality of macro-cells, micro-cells and/or pico-cells.
Specifically, for the exemplary mobile communications system shown in FIG. 1, in order for a user's mobile terminal to move from the public macro-cell 12 to any one of the private network cells 14, 16, 18 or 20, or from macro-cell 22 to private network cell 20, there has to be a neighboring-cell relationship defined between the macro-cell (12 or 22) and the respective private network cell (14, 16, 18 or 20). In addition, in order for a mobile terminal to move from a macro-cell to a private network cell, the mobile terminal must be able to "see" that private cell as a candidate for cell selection, and also be able to measure the signal strength of the base station transmitter that defines that private cell.
In certain mobile communications systems, such as, for example, the General Packet Radio Service (GPRS) of the digital cellular Global System for Mobile Communications (GSM), the mobile terminals are responsible for cell selection. Consequently, these mobile terminals must have access to all system parameters regarding which criteria to use for selecting a private cell while being located in a public macro-cell. As such, in such a system utilizing a hierarchical cell structure (HCS) with numerous private networks, there are large numbers of macro-cell neighbors, which creates the following problems.
First, in such conventional mobile communications systems as the GPRS, each corresponding set of cell selection parameters has to be broadcast on the control channel in the public macro-cell, which imposes an unnecessarily large load on that control channel. For example, in the GPRS, this unnecessarily large load is imposed on the Packet Broadcast Control Channel (PBCCH). As specified in the GSM Technical Specifications 03.64 and 05.08, there are 15 selection parameters associated with each neighboring cell, which corresponds to roughly 70 information bits. Consequently, since a load of roughly 700 information bits is a maximum acceptable load for the PBCCH, it is obvious that such a system operating with more than 10 neighbor cells will impose an unacceptable load on the PBCCH.
Second, in order to be able to cater to all of the conventional system's end users' needs, all available frequencies are broadcast over the Broadcast Control Channel (BCCH). Consequently, for cell selection purposes, the end users' terminals are required to make downlink signal strength measurements on all of those frequencies, including even those frequencies that correspond to private network cells which some (non-subscribing) users' terminals are unable to access. These unnecessary downlink signal strength measurements for non-subscribing terminals yields significantly low accuracies for measurements made on the other accessible cells.
Third, in such mobile communications systems as the GPRS, there is currently no means to communicate to any mobile terminal just what cells are accessible (public) and which are not (private). Consequently, even if the above-described problems were to be resolved, the existing systems provide no means to implement those solutions.
Another related problem found in conventional mobile communications systems is that private networks are made inaccessible to non-subscribing public end users in basically two ways. Either the private network is completely isolated from the public network, or the private network's cells are made part of the public network but marked with, for example, a "cell barred for public subscribers" flag in the broadcast system information given about those private cells. Obviously, a significant problem with totally isolating the private network from the public network is that no handovers can be made from a cell in the private network to a cell in the public network, or vice versa. Also, a significant problem with using the "cell barred" flag method is that all of the mobile terminals involved can "see" all of the available cells, including those cells which are inaccessible. In conventional mobile communications systems, the above-described problems lead to an unnecessary use of processing resources, less time available for mobiles to perform downlink measurements on true handover cell candidates, and/or unacceptably large loads imposed on the public network's control channels. However, as described below, these problems are successfully resolved by the present invention.